Astrophysics
Dark Matter and Dark Energy
Khadijeh Issa
03.06.2024
TH
12 GRADE SCIENCE
INTRODUCTION
Dark matter is a hypothesized kind of stuff that cannot absorb, reflect, or emit
electromagnetic radiation. This makes it extremely difficult to detect immediately with
existing technologies. It is "dark" not because it is black or lacks light, but because it does
not interact with light or other forms of electromagnetic radiation. In essence, it is
translucent, making it 'invisible' to our existing techniques of observation as it also
makes up almost 27% of the cosmos, outweighing visible matter roughly six to one.
Dark energy on the other hand is an unknown type of energy that permeates all of space
and appears to exert a repulsive pull, causing the cosmos to expand at a rapid speed.
Unlike matter, it does not clump under gravity and is scattered uniformly across the
cosmos.
History
Dark Matter
The dark matter idea originated in a discussion about the Earth's age. In 1846, British
physicist Lord Kelvin estimated the age of the Earth using thermodynamic rules. He
calculated that the Earth was between 20 and 100 million years old. Geologists and
evolutionary biologists had indicated hundreds of millions to billions of years, but this
was substantially younger. To explain this disparity, Kelvin proposed the existence of
"dark bodies" in the cosmos that altered Earth's temperature history via gravitational
pull. According to Kelvin, these objects might be stars that have cooled and faded to the
point of invisibility.
French scientist Henri Poincaré also explored the existence of dark matter in the cosmos.
In a 1904 lecture at the Congress of Arts and Sciences in St. Louis, Henri theorized about
"dark stars" that were unseen not because they were far away, but because they were
naturally dim. These unseen celestial bodies would exert tremendous gravitational force
on visible materials.
In 1932, Dutch astronomer Jan Oort studied the movements of neighboring stars in the
Milky Way. He discovered a difference between the galaxy's mass as deduced from the
number of stars and the mass computed from their speed. He postulated the possibility
of "dark matter" that we cannot see or detect using usual methods to account for this
discrepancy.
Fritz Zwicky's 1933 discovery confirmed the dark matter idea among the scientific world.
Zwicky investigated the Coma galaxy cluster and discovered that the galaxies inside it
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moved too quickly for the cluster's measured mass and should have flown apart. He
reasoned that there had to be some missing mass or dark matter holding the cluster
together.
Vera Rubin and Kent Ford's observations of galaxies' rotation curves in the 1970s
supported the dark matter concept. They discovered that galaxies were whirling so
quickly that they should have split apart in the absence of invisible matter's gravitational
attraction. Following decades of research and observations, dark matter became a key
component of our current cosmological theories.
Dark Energy
The origins of dark energy may be traced back to 1929, when American astronomer
Edwin Hubble suggested the expansion of the cosmos. Hubble discovered that galaxies
were moving away from one another, showing that the cosmos was not as static as
previously believed.
➔ Supernovae Observations
The late 1990s saw a watershed point in the development of the dark energy theory. Two
distinct teams of astronomers were examining Type Ia supernovae when they discovered
an incredible finding. They discovered that not only is the cosmos expanding, but the
pace of expansion is also increasing. This finding could not be explained just by gravity,
leading to the idea of dark energy as the driving factor behind this acceleration.
➔ The Cosmological Constant
Before Hubble's discovery, Albert Einstein's equations of general relativity included a
"cosmological constant" (Λ) to support a static universe, which was the prevalent view at
the time. When it became evident that the world was expanding, Einstein abandoned the
cosmological constant, dubbed it his "greatest blunder."
Properties of Dark Matter and Energy
Dark Matter
Most scientists believe that dark matter has the features described below, despite the fact
that its unique traits are being investigated.
★ Non- Baryonic: Dark matter is not composed of baryons, which are particles
found in regular matter like protons and neutrons.
★ Non-Luminous: It does not produce, reflect, or absorb light or other
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electromagnetic waves. It is invisible.
★ Gravity Interaction: Dark matter interacts gravitationally with normal matter and
light.
★ Collisionless: Dark matter particles do not interact with one another or with other
particles using strong or electromagnetic forces, therefore they pass straight
through each other and other particles.
Dark Energy
Dark energy is one of the most mysterious and interesting phenomena in current
astronomy.
★ It accounts for around 68% of the universe.
★ Is responsible for the cosmos's accelerated expansion.
Despite its importance in forming the universe, its nature remains mostly unknown and
unexplained.
Dark Matter vs Ordinary Matter and Antimatter
Everything visible, including stars, galaxies, planets, and even ourselves, is made up of
ordinary baryonic stuff. This stuff is made up of atoms, each of which contains protons,
neutrons, and electrons. Ordinary matter interacts with other matter using
electromagnetic forces, absorbing, emitting, and reflecting light. We identify its existence
with a variety of sophisticated devices.
In contrast, antimatter is a mirror image of regular matter. Its particles have the opposite
qualities as their matter counterparts. For example, a positron is an antimatter particle
that has the same mass as an electron but a positive charge. Matter and antimatter
destroy each other, producing energy.
Dark matter does not interact with electromagnetic forces in the same way that regular
matter and antimatter do. It does not emit, absorb, or reflect light and cannot be directly
observed. However, it interacts gravitationally with other stuff.
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Dark Energy vs Dark Matter vs Baryonic Matter
Dark energy should not be confused with dark matter, which comprises approximately
27% of the universe. Dark matter, unlike dark energy, has mass and clumps under
gravity, and it is principally responsible for galaxy formation and motion. While they are
both enigmatic and poorly understood, they play quite distinct functions in the universe.
Both dark energy and dark matter vary from baryonic matter, which makes up just
around 5% of the cosmos. Baryonic matter includes all of the usual matter we are
familiar with, such as protons, neutrons, electrons, and antimatter. The observable
cosmos is made up of baryonic matter, which includes the tiniest particles as well as
atoms, molecules, and celestial bodies.
Evidence for Dark Matter
★ Galactic Rotation Curves: According to physical rules, stars at the outer margins of
a rotating galaxy should travel slower than stars closer to the center. However,
studies reveal that stars near the periphery move just as swiftly, implying that
invisible mass (i.e., dark matter) influences their speed.
★ Gravitational Lensing: When light from distant galaxies passes close to big objects,
it bends owing to gravity. The term for this phenomenon is gravitational lensing.
Observations demonstrate that light bends more than predicted, implying the
presence of extra hidden mass.
★ The cosmic microwave background (CMB): the afterglow of the Big Bang. Detailed
CMB observations imply the presence of dark matter. The pattern of minuscule
temperature changes in the CMB shows that the cosmos is made up of around 5%
ordinary stuff, 27% dark matter, and 68% dark energy.
Theories About Dark Matter
★ Weakly Interacting Massive Particles (WIMPs): the most popular option. They are
hypothetical particles that interact poorly with conventional matter yet are
massive enough to explain the observable effects of dark matter.
★ Axions: hypothetical particles that are light, plentiful, and have weak interactions
with other particles, making them possible candidates for dark matter.
★ Sterile neutrinos: a potential form of neutrino that interacts with ordinary matter
even less than regular neutrinos do. They may be a possible source of dark
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matter.
★ Modified Newtonian Dynamics (MOND): This theory proposes changing the rules
of gravity at extremely large scales to explain the findings without using dark
matter.
★ Quantum Gravity and String Theory: Some theorists believe that a deeper
understanding of quantum gravity or the application of string theory might
answer the puzzle of dark matter. The gravitino is a hypothetical particle that
mediates supergravity interactions and is a candidate for dark matter.
Theories About dark energy
★ Cosmological Constant: An unchanging force that exists in the vacuum of space.
★ Quintessence: a dynamic field whose energy density can change over time.
★ Modified Gravity Theories: Alternatives to dark energy that modify general
relativity.
★ Extra Dimensions: Theories that interactions with higher-dimensional space might
result in cosmic acceleration.
Theories Explained
★ Cosmological Constant
One of the most straightforward theories for dark energy is the return of Einstein's
cosmological constant. In this scenario, the constant functions as a vacuum energy that
exists in space, producing a consistent, unchanging force that propels cosmic
acceleration.
★ Quintessence
According to Quintessence, dark energy is a dynamic field, as opposed to the
cosmological constant, which is constant. The energy density of quintessence can
fluctuate throughout time, potentially interacting with matter and other forms of energy
in the cosmos.
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★ Modified Gravity Theories
Some ideas propose that dark energy does not exist at all. Instead, changes to general
relativity, such as f(R) gravity, might explain the observed acceleration without requiring
an unknown type of energy.
★ Extra Dimensions
Theories incorporating additional dimensions, such as string theory, can provide other
theories. In these ideas, our universe may be a three-dimensional "brane" contained in a
higher-dimensional space, and interactions with this bigger "bulk" could explain the
observed acceleration.
Experiments
Dark Matter
★ Direct Detection Experiments
★ Indirect Detection Experiments
★ Collider Experiments
Dark Energy
★ Type Ia Supernovae Observations
★ Cosmic Microwave Background (CMB) Studies
★ Baryon Acoustic Oscillations (BAO)
★ Dark Energy Survey (DES)
★ Large Synoptic Survey Telescope (LSST)
★ Euclid Mission
★ WFIRST (Nancy Grace Roman Space Telescope)
★ DESI (Dark Energy Spectroscopic Instrument)
★ Quantum Experiments
Site:https://sciencenotes.org/dark-energy/
https://sciencenotes.org/what-is-dark-matter/
